1. BME 312 BIOMEDICAL INSTRUMENTATION II LECTURER: ALİ IŞIN
FACULTY OF ENGINEERING
DEPARTMENT OF BIOMEDICAL ENGINEERING
BME 312 BIOMEDICAL INSTRUMENTATION II LECTURER: ALİ IŞIN
LECTURE NOTE 3
BLOOD FLOW METERS
BME 312-BMI II-L3-ALİ IŞIN 2015
BME BMI II - ALİ IŞIN 2015

2. 1.Electromagnetic Blood Flow Meters
Measures instantaneous pulsatile flow of blood
Works based on the principle of electromagnetic induction
The voltage induced in a conductor moving in a magnetic field is proportional to the velocity of the conductor
The conductive blood is the moving conductor
BME 312-BMI II-L3-ALİ IŞIN 2015

3. Principle of Electromagnetic Blood Flow Meters
BME 312-BMI II-L3-ALİ IŞIN 2015

4. Principle of Electromagnetic Blood flow Measurement
BME 312-BMI II-L3-ALİ IŞIN 2015

5. BME 312-BMI II-L3-ALİ IŞIN 2015

6. Principle of Electromagnetic Blood Flow Meters
A permanent magnet or electromagnet positioned around the blood vessel generates a magnetic field perpendicular to the direction of the flow of the blood.
Voltage induced in the moving blood column is measured with stationary electrodes located on opposite sides of the blood vessel and perpendicular to the direction of the magnetic field.
BME 312-BMI II-L3-ALİ IŞIN 2015

7. Principle of Electromagnetic Blood Flow Meters
The Induced emf
Where
B = magnetic flux density, T
L = length between electrodes, m
u = instantaneous velocity of blood, m/s
BME 312-BMI II-L3-ALİ IŞIN 2015

8. Principle of Electromagnetic Blood Flow Meters
This method requires that the blood vessel be exposed so that the flow head or the measuring probe can be put across it.
BME 312-BMI II-L3-ALİ IŞIN 2015

9. Design of Flow Transducers
The electromagnetic flow-transducer is a tube of non-magnetic material to ensure that the magnetic flux does not bypass the flowing liquid and go into the walls of the tube.
The tube is made of a conducting material and generally has an insulating lining to prevent short circuiting of induced emf.
The induced emf is picked up by point electrodes made from stainless steel or platinum.
BME 312-BMI II-L3-ALİ IŞIN 2015

10. Design of Flow Transducers
The flow head contains a slot through which the intact blood vessel can be inserted to make a snug fit.
Several probes of different sizes must therefore accompany the flowmeter to match the full range of sizes of the blood vessels which have various diameters.
Flow heads having as small as 1mm are available.
BME 312-BMI II-L3-ALİ IŞIN 2015

11. Types of Electromagnetic Blood Flow Meters
DC Flow meters
Use DC Magnetic field.
Cause electrode polarization and amplifier drift.
AC Flow meters
Electromagnets are driven by alternating currents.
The transducer acts like a Transformer and induces error voltages that often exceed the signal levels by several orders of magnitude.
BME 312-BMI II-L3-ALİ IŞIN 2015

12. Electromagnetic AC flow meters
Error recovery is achieved by using several different waveforms for magnet current
Sine, Square, Trapezoidal.
Suitable balancing circuits are used to balance out the error voltage.
BME 312-BMI II-L3-ALİ IŞIN 2015

13. Sine wave Flowmeters
The transformer induced voltage is 90˚ out of phase and is eliminated by
Injecting a voltage of equal strength and opposite phase into the signal.
Using a gated amplifier.
Permit the amplification of the signal only during the flow induced voltages are maximum and the transformer induced voltages are minimum.
BME 312-BMI II-L3-ALİ IŞIN 2015

14. Square wave Flowmeters
The transformer induced voltage is only a spike.
Separation is easier as the amplifier can be gated only for a very short period.
Blanking is required only when the current in the magnet is reversing its direction and the amplifier works during the flat portion of the square wave.
BME 312-BMI II-L3-ALİ IŞIN 2015

15. Magnetic Flowmeter Block Diagram
BME 312-BMI II-L3-ALİ IŞIN 2015

16. Magnetic Flowmeter Block Diagram
The oscillator, which drives the magnet provides a control signal for the gate, operates at a frequency of between 60 and 400 Hz.
The frequency response is high enough to allow the recording of the flow pulses.
The mean or average flow can be derived by use of a low-pass filter.
BME 312-BMI II-L3-ALİ IŞIN 2015

17. 2. Ultrasonic Blood Flow Meters
A beam of ultrasonic energy is used to measure the velocity of flowing blood.
Two types:
Transit time flow meters
Doppler type.
BME 312-BMI II-L3-ALİ IŞIN 2015

18. Transit-Time Ultrasonic Flow Meters
Ultrasonic Transducer
Ultrasonic Transducer
BME 312-BMI II-L3-ALİ IŞIN 2015

19. Transit-Time Ultrasonic Flow Meters
Where
t - transit time
D - Distance between the transducers
c - Sound velocity
u - blood flow velocity
BME 312-BMI II-L3-ALİ IŞIN 2015

20. Transit-Time Ultrasonic Flow Meters
The pulsed beam is directed through a blood vessel at a shallow angle and its transit time is measured.
The transit time is shortened when the blood flows in the same direction as the transmitted energy
The transit time is lengthened otherwise.
BME 312-BMI II-L3-ALİ IŞIN 2015

21. Doppler Type Ultrasonic Flow Meters
BME 312-BMI II-L3-ALİ IŞIN 2015

22. Doppler type Ultrasonic Flow Meters
Based on the Doppler principle
A transducer sends an ultrasonic beam with a frequency F into the flowing blood.
A small part of the transmitted energy is scattered back and is received by a second transducer arranged opposite the first one.
The reflected signal has a different frequency F + FD or F – FD due to Doppler effect.
BME 312-BMI II-L3-ALİ IŞIN 2015

23. Doppler Frequency equation
Where
fd = Doppler frequency shift
f0 = source frequency
u = target velocity
c = velocity of sound
BME 312-BMI II-L3-ALİ IŞIN 2015

24. Doppler type Ultrasonic Flow Meters…
The Doppler component FD is directly proportional to the velocity of the flowing blood.
A fraction of the transmitted ultrasonic energy reaches the second transducer directly with the frequency being unchanged.
BME 312-BMI II-L3-ALİ IŞIN 2015

25. Doppler Type Ultrasonic Flow Meters
BME 312-BMI II-L3-ALİ IŞIN 2015

26. Doppler type Ultrasonic Flow Meters…
After amplification of the composite signal, the Doppler frequency can be obtained at the output of the detector as the difference between the direct and the scattered signal components.
For normal blood velocities, the Doppler signal is typically in the low audio frequency range.
BME 312-BMI II-L3-ALİ IŞIN 2015